Apparatus and method for non-invasive measurement of cardiac output

ABSTRACT

A comparatively light and compact permanent magnet arrangement for an MRI apparatus has a pair of opposed permanent magnet arrays with a shimming system to adjust the uniformity and strength of a magnetic field in a central chamber of the apparatus. The MRI apparatus is used to examine the extremities of a patient to determine cardiovascular characteristics from an analysis of the blood flow through selected arteries in the extremity. The information collected can be used to calculate such characteristics as total cardiac output, blood flow, arterial wall thickness and elasticity and the presence of plaque.

PRIORITY

This application is a division of U.S. patent application Ser. No.10/837,243, filed, Apr. 30, 2004 which claims priority from applicationSer. No. 60/467,015, filed 1 May 2003.

BACKGROUND OF THE INVENTION

This invention relates generally to the measurement of cardiac outputand, more particularly, to apparatus and methods for non-invasivelymeasuring cardiac output

Cardiac output is the volume of blood pumped by the heart during a giveninterval of time. Measuring cardiac output is a diagnostic techniqueused to evaluate cardiac function and detect the presence ofcardiovascular disease or abnormality. Normally, cardiac output variesto meet the body's demand for oxygen and for nutrition. The left side ofthe heart receives oxygenated blood from the lungs and contracts,pumping the blood through arteries to all parts of the body. Blood flowis affected by such factors as the contractility of the left ventricle,the resistance against which the heart must pump and the volume of bloodin the ventricle when pumping occurs.

Changes in cardiac output may indicate disease or may be evidence of anabnormality or change in a patient's cardiovascular system. Accordingly,measurement of cardiac output is a useful diagnostic tool and detectionof changes in cardiac output may allow a physician to investigate apatient's condition more thoroughly and take measures to prevent seriousor permanent damage.

Ideally, measurement of cardiac output should be quick, convenient, safefor the patient and should use equipment and techniques which arereliable, easy to operate and accurate.

There are a number of known techniques and devices for measuring cardiacoutput. One is the Fick method based upon work done by Adolph Fick inabout 1870. As performed today, the Fick method involves injecting ameasured amount of oxygen into an artery and then measuring theconcentration of oxygen at a selected site downstream of the injectionsite. Typically, such a procedure involves the insertion of twocatheters into a patient's body, one for the injection of the oxygen andthe other for the collection of blood which is then analyzed and theoxygen content determined. While this technique produces satisfactorilyaccurate results, it does require surgically invasive procedures and theremoval of an appreciable amount of blood which must then be analyzed,resulting in a measurement that is not a real time measurement. Wheneveran invasive procedure is performed, it carries with it the attendantrisk of possible infection.

In another technique for measuring cardiac output a dye is injected intothe bloodstream and the dye concentration is measured at a pointdownstream of the injection site. Alternatively, a bolus of a chilledfluid indicator is injected and the temperature of blood withdrawn froma site downstream of the injection site is measured.

These and other cardiac output measurement techniques are wellrepresented in the prior art.

The following references are representative of known invasivemeasurement techniques.

U.S. Pat. No. 5,797,395 (Martin) teaches and describes a continuouscardiac output derived from arterial pressure wave form using patternrecognition. Martin uses a fluid catheter installed in the bloodstreamto measure the arterial pressure of a patient and to digitize the datacollected by the sensors. The collected data is then compared torepresentative data stored in a database with representative wave formscorresponding to various levels of cardiac output.

U.S. Pat. No. 6,299,583 (Eggers) teaches and describes monitoring totalcirculating blood volume and cardiac output. Eggers et al uses avariation of the indicator dilution technique in which a test substanceor analyte is injected into the body. As seen in FIG. 1 of Eggers et al,analyte is injected into the bloodstream by way of a catheter insertedinto the subclavian vein leading to the heart. A catheter with a sensoris inserted into, for example, the radial artery in one of the patient'sarms and is used to detect and measure the concentration of analyte.

U.S. Pat. No. 6,186,956 (McNamee) teaches and describes a method andsystem for continuously monitoring cardiac output without having toinject substances into the patient's body nor withdraw liquids from thebody. McNamee does so by using a pressure transducer placed in apatient's mouth or a tracheal cannula in measuring the differential inpressure as the patient breathes in and out. The detection of thissignal is then correlated to a table of known values from which cardiacoutput is deduced. McNamee references the use of MRI as an acceptablenon-invasive measurement technique and also states that such techniquestypically require large, expensive equipment and highly trainedtechnicians to operate the equipment and interpret the results.

U.S. Pat. No. 6,322,518 (Young et al) teaches and describes a method andapparatus for measuring cardiac output which utilizes an esophagealprobe inserted down the patient's throat to measure thoracic impedancevariations which are then correlated to a measurement of cardiac output.

Even minimally invasive procedures require skilled technicians, riskinfection and allergic responses, and cause apprehension and discomfortin the patient. The advantages of using non-invasive techniques formeasuring cardiac output have been recognized in the prior art.

U.S. Pat. No. 6,306,098 (Orr et al) teaches and describes apparatus andmethod for non-invasively measuring cardiac output. Orr et al use amodification of the Fick approach, measuring the difference in carbondioxide concentration between the air inhaled and exhaled by a patientthrough a breathing tube rather than measuring concentration of oxygenat disparate points in the patient's bloodstream. A computer program isused to calculate cardiac output based upon the differential in carbondioxide concentration.

U.S. Pat. No. 5,458,126 (Cline et al) teaches and describes a cardiacfunctional analysis system employing gradient image segmentation. Clineet al describe the use of techniques such as computed axial tomography(CAT) and MRI to create a four-dimensional data set to create images ofselected portions of the cardiovascular system. Cline et al arerepresentative of the use of complex, expensive and quite largeequipment to carry out these measurements.

U.S. Pat. No. 5,417,214 (Roberts et al) teaches and describesquantitative blood flow measurement using steady-state transport inducedadiabatic fast passage. Again, Roberts et al is representative of thecomplexity presently experienced in the use of MRI to carry outcardiovascular measurements and evaluation. In particular, at column 5,lines 26 et seq. Roberts et al describe the necessity for precisetechniques for gathering the data used in the analysis.

U.S. Pat. No. 6,348,038 (Band et al) teaches and describes method andapparatus for the measurement of cardiac output using the wave formcreated by the measurement of arterial blood pressure in a patient. Thecollected data is used to determine nominal stroke volume and using thisdetermination to obtain a nominal value for the patient's cardiacoutput.

U.S. Pat. No. 5,360,005 (Wilk) teaches and describes a medical diagnosisdevice for sensing cardiac activity and blood flow. Wilk uses anacousto-electric transducer as a type of stethoscope to convert heartsounds into electric impulses and then uses a microprocessor to analyzethese impulses and correlate them to cardiovascular activity.

U.S. Pat. No. 4,509,526 (Barnes et al) teaches and describes method andsystem for non-invasive ultrasound Doppler cardiac output measurement.Barnes et al teach the positioning of a transducer at separate locationson a patient's body to determine ultrasonically the cross-sectional areaof a patient's ascending aorta and to determine the systolic velocityprofile of blood flow through the aorta, enabling the calculation ofcardiac output. To practice the technique described in Barnes et al thetransducer must be accurately positioned by a technician at two separatesites in order to generate the data required to make the calculationsnecessary to determine cardiac output.

U.S. Pat. No. 5,178,151 (Sackner) teaches and describes a system fornon-invasive detection of changes of cardiac volumes and aortic pulses.Sackner et al use transducers positioned on the patient's body to sendand receive signals which include a wave form characteristic of theventricular volume and then continuously monitors to detect changes inthe wave form.

U.S. Pat. No. 5,443,073 (Wang et al) teaches and describes a system andmethod of impedance cardiography monitoring using body-mountedelectrodes to collect EKG signals which are then mathematicallyprocessed to predict stroke volume.

U.S. Pat. No. 5,423,326 (Wang et al) teaches and describes apparatus andmethod for measuring cardiac output in which transducers are attached toa patient's body and the signals generated and collected by thesetransducers are transformed and analyzed to correlate these signals tocardiac output.

U.S. Pat. No. 5,309,917 (Wang et al) teaches and describes a system andmethod of impedance cardiography and heartbeat determination using acombination of thoracic impedance and electrocardiogram signals with thedata represented by these signals processed to determine suchcharacteristics as stroke volume and cardiac output.

U.S. Pat. No. 5,685,316 (Schookin et al) teaches and describes anon-invasive monitoring of hemodynamic parameters using impedancecardiography. Bioimpedance electrodes are used to collect data across apatient's thoracic region and the data so collected are analyzed andcorrelated to heart stroke volume from which cardiac output iscalculated.

Although diagnostically useful, the foregoing art references demonstrateserious drawbacks ranging from the size, complexity and expense of theequipment used to obtain information to the requirement that theinstruments used to collect the data. As an example, thoracic impedancesystems use EKG leads which must be accurately positioned on andattached to a patient's body in order to effect the collection of datathat is later translated into values for cardiac output.

In a text entitled “Magnetic Resonance of the Heart and Great Vessels”,edited by J. Bogaert, A. J. Duerinckx and F. E. Rademakers(Springer-Verlag, Berlin 2000) the principles of MRI as applied tocardiology are set forth in Chapter 1, entitled “Techniques for CardiacMRI”, written by H. Bosmans. MRI examination begins with placing thepatient within a strong, static magnetic field which aligns the spins ofthe protons contained in body tissues and fluids. Thereafter a radiofrequency (RF) pulse is used to excite the protons and disturb thealignment of the protons with the magnetic field. A coil or detector isused to capture the signal produced by this change in alignment andreturn to the original alignment. The signal is influenced by two“relaxation times”, T1 and T2, generally described as measuring thetimes it takes the longitudinal and transverse components of the“excited” protons to return to the aligned state. The resulting signalsare characteristic of the type of tissue being examined and areprocessed to create an image of the tissues being studied.

The use of MRI in cardiac examination is also described in the March2001 issue of Medica Mundi in an article reprinted from “Heart Care”written by E. Nagel and E. Fleck, beginning at page 23. At page 29 ofthe article, the use of MRI to determine blood flow velocities isdiscussed and the need for more accuracy in the equipment used isidentified.

MRI technology is used to diagnose coronary disease by examining theflow of blood through blood vessels located in the arms. In one suchproposed technique, the flow of blood is occluded with an inflatablecuff and MRI is used to measure the dilation of the blood vessels andthe shear stress generated by the force of blood flow against theendothelial cells. Under normal circumstances, the endothelial cellswill produce the chemical nitric oxide to cause the coronary arteries towiden thus increasing blood flow to the heart and other muscles duringtimes of stress. In a patient with coronary artery disease this effectmy be greatly attenuated.

Accordingly, the need exists for apparatus and methods to determinecardiac output which are wholly non-invasive, require no leads or otherdata collection devices to be attached to the patient, utilizerecognized accurate diagnostic techniques such as MRI and which may beused in obtaining quick and accurate measurement of cardiac outputwithout requiring significantly complex placement or measurementtechniques.

A further object of the present invention is to provide an MRI unitwhich is relatively small and inexpensive yet which provide theprecision and resolution necessary to make accurate measurements ofcardiac output.

A further object is to make such equipment easy to operate without theuse of time-consuming or complex diagnostic techniques.

A further object is to provide methods for the use of such equipment tomake the required measurements carrying out such measurements on apatient's extremities rather than the thoracic cavity or the heartitself.

A further object is to avoid the use of drugs, analytes, dyes, tracersand other foreign substances.

The present invention is concerned with apparatus and methods to carryout the analysis of blood flow, preferably in the radial and ulnararteries. The apparatus consists of an MRI unit significantly smaller insize than the full-body MRI units presently used to conductcardiovascular examination. A pair of opposed magnet assemblies joinedby a magnetically conductive yoke defines an air gap between theassemblies. When a patient's arm is placed within the air gap the unitis used to carry out MRI analysis of the blood vessels in the arm.Internal and peripheral shimming mechanisms allow the uniformity of themagnetic field to be adjusted, and the configuration of the magnetscreates a magnetic field of about 1.0 tesla. A preferred configurationof the apparatus will be approximately 60 cm. by 60 cm. incross-sectional size and will weigh about 250 kg.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further objects of the present invention will become moreapparent upon consideration of the accompanying drawings wherein:

FIG. 1 is an illustration showing use of apparatus embodying the presentinvention to examine the arm of a supine patient;

FIG. 2 is an illustration showing use of the present invention toexamine the arm of a patient seated at a desk;

FIG. 3 is a partial sectional view of a first permanent magnetarrangement designed for use with the present invention;

FIG. 4 is a view along 4-4 of FIG. 3;

FIG. 5 is a partial sectional view of a second permanent magnetarrangement designed for use with the present invention;

FIG. 6 is an enlarged sectional view of an adjustable pole piece shimshown in FIG. 5;

FIG. 7 is a view along 7-7 of FIG. 5;

FIG. 8 is a cross-section of a patient's arm illustrating measurement ofthe radial and ulnar arteries;

FIG. 9 is a schematic view of an RF coil array used with the presentinvention;

FIG. 10 is a cross-sectional view of a sleeve worn by a user of thepresent invention;

FIG. 11 is a lateral view of a second embodiment of the pole piece ofFIG. 1;

FIG. 12 is a plan view of the cubic elements making up the pole piece ofFIG. 11;

FIG. 13 is a partial view of a pole piece segment; and

FIG. 14 is a partial lateral view of a second embodiment of the shimrings of FIG. 1.

While the following describes a preferred embodiment or embodiments ofthe present invention, it is to be understood that this description ismade by way of example only and is not intended to limit the scope ofthe present invention. It is expected that alterations and furthermodifications, as well as other and further applications of theprinciples of the present invention will occur to others skilled in theart to which the invention relates and, while differing from theforegoing, remain within the spirit and scope of the invention as hereindescribed and claimed. Where means-plus-function clauses are used in theclaims such language is intended to cover the structures describedherein as performing the recited functions and not only structuralequivalents but equivalent structures as well. For the purposes of thepresent disclosure, two structures that perform the same function withinan environment described above may be equivalent structures.

Referring now to FIG. 1, the numeral 10 indicates generally a diagnosticapparatus consisting generally of a cabinet 12 within which a permanentmagnet assembly is disposed. Access to the cabinet is via a port 14through which a patient 16 inserts his or her arm to place the armwithin a magnetic field created by the magnet assembly. As seen in FIG.2, the same apparatus can be used for a patient 18 when in the supineposition. Apparatus 10 is relatively lightweight, compact in size andcapable of providing real-time data.

Referring now to FIG. 3, the numeral 20 indicates generally a sectionalview of a first permanent magnet assembly designed for use in diagnosticapparatus 10.

Assembly 20 consists of a steel main magnet 22 formed as a solid rightcircular cylindrical section having a vertical axis A. A circulartoroidal side magnet 24 is positioned coaxially with main magnet 22about axis A and in this preferred embodiment has a rectangular orsquare cross-section. A steel pole piece 26, formed as a solid rightcylindrical section equal in diameter to main magnet 22 is positioned inface-to-face contact with main magnet 22 at main magnet face 28. Thisentire assembly is positioned within a hollow steel sleeve 30 which isclosed off by a steel end cap 32.

In a preferred embodiment of the present invention, first permanentmagnet assembly 20 is placed in face-to-face relationship with anidentical permanent magnet assembly 34 having identical components toand spaced apart from first magnet assembly 20 to form an air gap 36therebetween. Sleeve 30 extends to enclose magnet assemblies 20, 34 andair gap 36. A pair of removable side walls 38, 40 allow access to airgap 36 when removed from sleeve 30.

Permanent magnet assemblies 20, 34 are opposite in polarity to create amagnetic flux field across air gap 36. A centrally located segment ofair gap 36 is identified in FIG. 3 as diagnostic zone 42 across whichthe magnetic flux is at its most powerful and its most uniform.

Magnet assembly 20 includes means for adjusting the flux field acrosszone 42. In a first embodiment of magnet assembly 20, pole piece 26 hasa first centrally located cylindrical cavity 44 formed in interior face26 a. A second, circular cavity 46 is formed coaxial with cavity 44 anda third circular pole piece cavity 48 is similarly formed, coaxial withcavities 44 and 46.

Disposed within cavity 44 is a first cylindrical steel shim 50. Disposedwithin second circular cavity 46 is a toroidal steel shim 52 anddisposed within third circular cavity 48 is a second toroidal steel shim54.

Each shim 50, 52, 54 is adjustable along an axis parallel to axis A ofpermanent magnet assembly 20. A preferred embodiment of the adjustingmechanisms for shim 54 is shown in FIG. 3. A shaft 56 is bored throughterminating in a countersink 58 larger in diameter than that of shaft 56and extending to face 60 of pole piece 26. A threaded fastener, such asa bolt 62, is journalled to shim 54 and shim 54 may be adjusted withrespect to face 60 by turning bolt 62 to move shim 54 toward or awayfrom face 60.

At least three such bolts are attached to shim 54 and are placedequidistantly about the circumference of shim 54.

FIG. 4 is a view taken along 4-4 of FIG. 3 and illustrates thepositioning of the shafts and adjusting bolts for each of the shims. InFIG. 4, bolts 62, 64 and 66 are attached to and serve to adjust shim 54.In like fashion, bolts 68, 70 and 72 are attached to and work to adjustfirst shim 52. Bolts 74, 76 and 78 are journalled to and serve to adjustcylindrical shim 50.

In a preferred embodiment of magnet assembly 20, a second set oftoroidal steel shims are provided for the purpose of strengthening andadjusting the linearity of the magnetic flux field across air gap 36.

As seen in FIG. 3, steel sleeve 30 has a circumferentially extendingchannel 80, a concentric set of toroidal steel shims 82, 84, 86 arepositioned in channel 80 and are supported and adjusted by a series ofadjusting bolts such as those described in connection with shims 50, 52and 54. As seen in FIG. 3, a bore 88 is formed extending through end cap32 and having a countersink 90 formed at the end thereof. An adjustingbolt 92 is threadedly journaled to sleeve shim 82. A minimum of threesuch adjusting bolts are provided for each said shim and, as seen inFIG. 4, identical adjusting bolts are provided for sleeve shims 84 and86. FIG. 4 shows, in section, bolts disposed in bores such as bore 88,with bolts 92, 94 and 96 attached to shim 84, bolts 98, 100 and 102attached to shim 84 and bolts 104, 106, and 108 attached to shim 86.

Referring now to FIG. 5 the numeral 110 indicates generally a sectionalview of a second permanent magnet assembly designed for use indiagnostic apparatus 10.

Magnet assembly 110 consist of a steel main magnet 112, formed as asolid right circular section having an axis B. A circular toroidal sidemagnet 114 is positioned coaxially with main magnet 112 about axis B. Inthe embodiment shown herein, side magnet 114 has a square cross sectionalthough cross sections of varying shapes may be selected. A steel polepiece 116, formed as a right solid cylindrical section equal in diameterto main magnet 112 is positioned in face-to-face with main magnet 112and main magnet face 118. The entire assembly is positioned within aright cylindrical steel sleeve 120 which is closed off by a cylindricalsteel end cap 122.

As with first magnet assembly 20, and consistent with a preferredembodiment of the present invention, second permanent magnet assembly110 is placed within sleeve 120 in face-to-face relationship with anidentical, mirror image permanent magnet assembly 124 having identicalcomponents and construction and spaced apart from second magnet assembly110 to form an air gap 126 therebetween. Steel sleeve 30 extends toenclose magnet assemblies 110, 124 and air gap 126.

A pair of removable side walls 128, 130 allow access to air gap 126 whenremoved from sleeve 120. Permanent magnet assemblies 110, 124 areopposite in polarity to create a magnetic flux field across air gap 126.A centrally located segment of air gap 126 is identified in FIG. 5 as adiagnostic zone 132 across which the magnetic flux is at its mostpowerful and uniform. For purposes of illustration, face 118 of mainmagnet 112 has a north polarity while the corresponding face of the mainmagnet in magnet assembly 124 has a south polarity. As with magnetassembly 20, magnet assembly 110 includes means for adjusting thelinearity of the flux field across air gap 126. In this second permanentmagnet assembly embodiment, 110, exterior face 116 a of pole piece 116has a first centrally located cylindrical cavity 134 formed therein. Asecond, circular cavity 136 is formed coaxial with cavity 134 and athird circular pole piece cavity 138 is similarly formed, coaxial withcavities 134 and 136.

As seen in FIG. 5, a first steel shim 140 is disposed within cavity 44.Shim 140 is formed as a solid right cylindrical section or disk.Disposed within second circular cavity 136 is second steel shim 142formed as a toroid and disposed with a third circular cavity 138 is athird steel shim 144 also formed as a toroid. In the preferredembodiment shown in FIG. 5, shims 142 and 144 are formed with square orrectangular cross-sectional shapes although other shapes may be selectedas found desirable or necessary.

Each shim 140, 142 and 144 is adjustable along an axis parallel to axisB of permanent magnet assembly 110. A preferred embodiment of theadjusting mechanism for shim 144 is shown in FIG. 6. Shim 144 has aseries of tapped or threaded apertures 146 formed parallel to axis B. Ina preferred embodiment, apertures 146 are formed midway to the inner andouter diameters of shim 144 and at regularly spaced intervals. Forexample, three such apertures may be formed displaced one from the otherby an angle of 120 degrees. An adjusting screw 148 has a head 150 and ashaft 152, with shaft 152 sized and threaded to engage tapped aperture146. End 154 of shaft 152 is rotatably journalled to and supported by asupport 156, allowing screw 148 to rotate in either a clockwise orcounterclockwise direction. As screw 148 is rotated, shim 144 travelsalong shaft 152 as shaft 152 threads along tapped aperture 146.

Referring again to FIG. 5, a series of sleeve shims 252, 254 and 256 areshown concentrically disposed in a circumferentially extending channel258 formed in sleeve 120. Shims 252, 254 and 256 are axially adjustablein the same manner as sleeve shims 82, 84 and 86 shown in FIG. 3. Arepresentative threaded adjusting bolt 260 passes through a threadedbore 262 with bolt head 264 seated in countersink 266. Bolt end 268 isjournaled to sleeve shim 252 in the same manner as shown in FIG. 6 andshim 252 is adjusted axially by threading bolt 260 along bore 262.Preferably, at least three such adjusting bolts are provided for eachsleeve shim, spaced at regular intervals.

Referring to FIG. 7, shim 144 is shown in cavity 138 and supported byadjusting screws 148, 158 and 160. Similarly, shim 142 is shown incavity 134 and supported by adjusting screws 162, 164 and 166. Centraldisk shaped shim 140 is shown in cavity 132 and is supported byadjusting screws 168, 170 and 172.

When assembled, the pole piece shims and annular shims described aboveare adjusted to produce a magnetic field across air gap 36 and air gap126 with a high degree of uniformity in field strength and fielddirection.

Apparatus 10 is used to practice non-invasive methods of assessing totalcardiac output, relative cardiac output, arterial wall thickness andelasticity and flow mediated dilation. Measurement of these functions iscarried out non-invasively on a real-time basis and in a setting whichis convenient and comfortable for the patient. These measurements areused for initial examinations and to track changes in the circulatorysystem before and after a given treatment. Use of apparatus 10 incarrying out MRI examinations is not subject to the variations andresults produced using such techniques as ultrasound where personnelmust be highly trained to position accurately the probe used duringexaminations.

In a preferred embodiment of the present invention, apparatus 10 is usedto perform an MRI examination of the radial and ulnar arteries in apatient's arm.

Referring now to FIG. 8, the numeral 174 identifies the arm of a patientshown in a schematic-type cross-section. For the purposes ofillustrating the present invention, reference will be made to thepermanent magnet arrangement shown in FIG. 3. Arm 174 is placed withinapparatus 10 as shown in FIG. 1 or 2 with arm 174 extending through airgap 36 and positioned within diagnostic zone 42. An RF coil 176 ispositioned above arm 174 and a gradient or pickup coil 178 is positionedbeneath arm 174 to receive the signal created when RF coil 176 ispulsed.

As seen schematically in FIG. 8, the radial and ulnar bones 180, 182 areshown, as are the radial and ulnar arteries 184, 186. As the heartbeats, the blood pressure within the body's arterial blood vesselsvaries from a maximum (systolic) and minimum (diastolic) value. Thesystolic blood pressure represents pressure within the blood vesselduring a heartbeat while the diastolic pressure is that measured whenthe heart is at rest between beats. Arteries 184, 186 are shown duringthe diastolic portion of the heartbeat cycle. When the heart beats,arteries such as 184, 186 expand as shown at 188, 190.

The apparatus and methods of the present invention measure and recordthe changes in size of arteries 184, 186 with the heartbeat and betweenheartbeats. In this fashion, the cross-sectional area of arteries 184,186 can be determined. It is also known to determine the rate of flow ofblood using commonly applicable MRI techniques and the combination ofthe determination of artery cross-sectional area and rate of flow allowscalculation of the portion of the total cardiac output passing throughthe representative arteries during a heartbeat.

This information can be used to establish a baseline cardiac output fora patient with any deviation in output over time operating as anindicator that the patient's cardiac condition has changed. Cardiacoutput can also be calculated by using known values of the percentage ofcardiac output normally passing through the radial and ulnar arteriesand using this ratio to estimate total cardiac output. For the purposesof this description, total cardiac output is defined as the volume ofblood pumped from the heart with each beat, approximately 12% of whichmoves through the brachial arteries of each arm with each beat and thedetected blood flow volume can be compared to tables of known blood flowratios for patients of different ages, weights and other variables.

MRI allows a cross-section to be taken that includes both arteries 184,186 simultaneously. In order to create a high resolution image, it maybe necessary to reposition the arm within the magnetic field prior topulsing the RF electrodes. An alternative to physical repositioning isthe use of an array of electrodes and a computer program to adjust thepulsing of the electrodes to produce improved images.

For purposes of accuracy, pick up coil 178 should be positioned as closeas possible to the arteries being measured. Use of the relatively smalldiagnostic area 42 makes this possible and readings can be taken withthe patient's arm extending into apparatus 10 with the palm eitherfacing in the upward or downward position or rotated to produce anoptimum image of the arteries.

Referring now to FIG. 9, the numeral 192 identifies generally an arrayof RF coils positioned around a patient's arm 194. RF coils such asthose used as electrodes in MRI diagnostic equipment can be used aspulsing or receiving coils or can function as both pulsing and receivingcoils. For a single coil to both pulse and receive, an rf signal ispulsed through the coil. This transmission is gated and after it hasceased the coil goes through a period of quiescent or “dead” time whereno signal is being sent nor received. Thereafter, the coil is activatedto receive the data produced by its own pulsed signal. The data signalis also gated to create a second period of dead time, after which thecoil may then again be pulsed to transmit another signal.

As seen in FIG. 9, an array of RF coils 196, 198, 200 are positionedequidistantly about the circumference of array perimeter 202. It ispossible to pulse these coils in sequence to create a “virtual coil”effectively positioned at selected points about the periphery ofperimeter 202. It is also possible to mechanically rotate perimeter 202to bring coils 196, 198, 200 into a different physical location withrespect to arm 194. By performing actual or virtual rotation of thepositions of coils 196, 198, 200 and selectively using each coilselectively to pulse energy, receive energy or both it is possible toadjust device 10 to produce the clearest possible image of arm 194.

To increase the sensitivity and adjustability of array 192, additionalRF coils can be added to the array such as coils 204, 206, and 208 andto control the pulsing and collection capabilities of each coil bycomputer in order to adjust the received image.

Referring now to FIG. 10, the numeral 210 identifies a cross-sectionalview of a tubular sleeve within which RF coils 212, 214 and 216 aredisposed. Sleeve 210 can be formed of fabric and is intended to beslipped over the patient's arm or other appendage and to thereafter beplaced within device 10. Sleeve 10 allows coils 212, 214, 216 to beplaced as close to the area of examination as possible and to provide auniform dispersion of signal transmission and collection independent ofthe placement of coils within device 10 itself.

As described in connection with array 192, sleeve 210 can include anyselected number of coils and the arrangement shown in FIG. 10 is by wayof example only.

Sleeve 210 can be formed as a permanent, flexible and expandable cuff orcan be formed as a disposable unit which can be discarded after use.Sleeve 210 can be provided in a range of sizes to accommodateextremities of varying proportions, can be written upon to recordpatient information, and can contain various numbers and configurationsof coils and coil leads that can selectively be connected to provideenergy to selected coils.

Referring now to FIG. 11, an alternative construction for pole piecessuch as pole piece 60 is shown. FIG. 11 is a cross-sectional view of apole piece 218 constructed as a series of laminae 220, 224, 226, 228 and230 glued together in a solid array by non-conductive epoxy glue orother well-known permanent adhesives.

As seen in FIG. 12, a section of laminae 220 is shown in partialperspective. Each laminae is formed of a series of cubes 232 preferablyextruded from ferromagnetic material and glued together with theadhesives described above. In a preferred embodiment of the invention,each cube is approximately 10 mm by 10 mm by 10 mm. When cubes 232 havebeen solidly glued into a planar array, each such array is stacked andglued to corresponding arrays to produce the structure shown in FIG. 11.The completed array may then be machined to the shape desired to use thecompleted array as a pole piece.

Use of cubic elements are believed to limit the eddy currents createdwithin the ferromagnetic pole piece when the RF coils are pulsed.Referring now to FIG. 13, a top plan view of a segment of pole piece 218is shown illustrating the appearances of cubes 232 when cemented intothe array and shaped to be used as a pole piece. In assembling alaminated pole piece such as 218, cubes 232 are arranged such that theinterfaces of adjacent cubes are offset from the interfaces of the cubesin the layers immediately above and below thereby adding strength to thearray.

Referring now to FIG. 14, the numeral 234 identifies a segment of analternative construction of the torroidal shim rings such as 54 of FIG.3. The embodiment of FIG. 14 shows shim 234 assembled from overlappingsegments 236, 238 with segment 236 having an upper edge 240 andintermediate land 242 and a lower edge 244 while segment 238 has anupper edge 246 an intermediate land 248 and a lower edge 250. Whenassembled, edges 240, 246 abut as do lands 242, 248 and lower edges 244,250. At each of these abutments, an epoxy or other suitable adhesive isused to permanently attach the segments together.

It is also contemplated that the torroidal and disk-shaped shimsdescribed earlier can also be constructed from laminae in the mannershown in FIGS. 11, 12, and 13. In such constructions, it is contemplatedthat cubes 232, when used in shim constructions, can be extruded fromferromagnetic metals, non-ferromagnetic metals, non-metallic substancesor magnetic material.

While not herein specifically shown, it is acknowledged that the use ofelectromagnetic gradient coils as shims to adjust the linearity andshape of the magnetic field of a permanent magnet array is well knownand can be included in the arrangements described herein. Such shimcoils can also be used as gradient coils by pulsing the coils tointentionally distort the field of view to aid in distinguishing betweentissues of different types. Preferably, separate shim and gradient coilsare used to keep voltage to the shim coils constant.

The disclosed methods and apparatus offer advantages over prior knownMRI diagnostic methods in that a relatively small permanent magnetarrangement is used to produce a high strength uniform and highresolution magnetic field across a very small part of the body. This isa much different approach than that described in the Nagel and Fleckarticle referenced above in which a patient is “positioned within thebore of a cylindrical superconducting magnet.” The strength of themagnetic field produced by the present invention is estimated in excessof 1.0 tesla and the relatively small size of the body portion beingsampled creates an image with a higher signal-to-noise ratio than can beachieved when the entire body is placed within a magnetic field. Theresulting high resolution images allow the changes in size of thearteries to be accurately determined.

When performing MRI diagnostic procedures, care must be taken to avoiddamage to nerves caused by too high a field gradient across the area ofthe body being examined. For the purposes of this description, the fieldgradient is described as the strength of the magnetic field divided bythe area of the field of view in question. Using the greatly reducedfield of view made possible by the present invention, allows for highergradients to be safely used when examining patients. This results inhigher resolution images and more accurate diagnostic information.

Total cardiac output can be estimated by relating the radial and ulnararteries and comparing these measurements to known values of percentagesof cardiac output measured through such arteries.

Use of the present invention thus allows cardiac functions to beexamined in a strong, small magnetic field with a relatively high fieldgradient. This will also result in the capability of using rapid pulsesto produce accurate images showing the changes in size of the arteriesin question.

The apparatus described herein uses less energy and is less expensive tobuild than the presently known full body type MRI diagnostic units.

In evaluating cardiac function, the RF pulses may be timed to commencewith the heartbeat and can be used to track changes in the heartbeat.Alternatively, use of the present invention may be used without usingthe heartbeat as a trigger or marker with the apparatus being operatedthrough a sufficient number of cycles to guarantee that an entire cyclehas been captured and accurately characterized.

Use of MRI to create images of arteries 186, 188 is superior to the useof ultrasound because MRI produces a cross-sectional view which allowsthe system to track the changes in size and configuration of arteries186, 188. Ultrasound, on the other hand will provide only a lateral viewand even though this lateral view can show changes in the apparentdiameter of the arteries, it does not provide an accurate view of theactual cross-sectional configuration of the artery. Where an artery is,for example, not perfectly round or in some way impeded, thecross-section will be less than circular. However, MRI will enable theoperator to determine the exact shape and, therefore, area of thecross-section and to calculate the blood flow therethrough.

What is claimed is:
 1. A method for non-invasively performing anexamination to determine selected physical characteristics of the heartand cardiovascular system of a living subject, said method comprisingthe steps of: selecting an extremity of the subject suitable forconducting said examination; providing an apparatus adapted to conductMRI analysis of selected blood vessels within said extremity, said MRIapparatus of the type having an integral housing enclosing first andsecond opposed permanent magnet arrangements defining an air gap therebetween, an access port in said housing to enable the placement of saidextremity within said air gap, and means to create a magnetic fieldacross said air gap with sufficient field strength, uniformity andresolution to perform said examination; providing said apparatus withside magnets and at least one shim magnet adapted to increase thestrength and uniformity of said magnetic field, said shim magnetprovided with at least one adjusting bolt rotatably journaled to each ofsaid shim magnet; wherein adjustment of said bolt results in a change insaid magnetic field such that a uniform magnetic field is provided;providing an array of RF coils to pulse and detect electromagneticsignals within said air gap; pulsing selected of said RF coils anddetecting the signals produced thereby; rotating said adjusting bolt;thereby increasing the strength and uniformity of said magnetic field;creating an MRI image of said selected blood vessels at selected timeintervals to produce information to determine said heart andcardiovascular system characteristics; and collecting and analyzing saidinformation; wherein said heart and cardiovascular systemcharacteristics are determinable accurately within a high strengthuniform and high resolution magnetic field across a portion of the body.2. The method as recited in claim 1, additionally comprising a step ofselecting said heart and cardiovascular system characteristics to be thecross-sectional area of a selected blood vessel measured at selectedtimes during the beating of said heart.
 3. The method as recited inclaim 1, additionally comprising a step of selecting said heart andcardiovascular system characteristics to be the flow rate of bloodthrough a selected blood vessel measured at selected times during thebeating of said heart.
 4. The method as recited in claim 1, additionallycomprising a step of selecting said heart and cardiovascular systemcharacteristics to be the total cardiac output of said heart.
 5. Themethod as recited in claim 1, additionally comprising a step ofselecting said heart and cardiovascular system characteristics to be thewall thickness of a selected blood vessel.
 6. The method as recited inclaim 1, additionally comprising a step of selecting said heart andcardiovascular system characteristics to be the elasticity of a selectedblood vessel.
 7. The method as recited in claim 1, additionallycomprising a step of selecting said heart and cardiovascular systemcharacteristics to be the flow mediated dilation of a selected bloodvessel.
 8. The method as recited in claim 1, additionally comprising astep of selecting said heart and cardiovascular system characteristicsto be the presence, type and thickness of plaque in a selected bloodvessel.
 9. The method as recited in claim 4, additionally comprising astep of selecting said extremity to be a human arm and selecting saidblood vessels to be the radial and ulnar arteries in said arm, andcalculating total cardiac output by determining the volume of blood perunit time flowing through said arteries and comparing said blood volumeto tables of known relationships of total cardiac output to blood flowmeasured through said arteries.
 10. The method as recited in claim 1,additionally comprising a step of pulsing said coils when said heartbeats.
 11. The method as recited in claim 1, additionally comprising astep of performing a first said examination for selected of saidphysical characteristics and storing the results of each saidexamination for use as a base line to determine subsequent changes insaid characteristics.